Induction hardening is a method of quickly and selectively hardening the surface of a metal part. A copper coil carrying a significant level of alternating current is placed near (not touching) the part. Heat is generated at, and near the surface by eddy current and hysteresis losses. Quench, usually water-based with an addition such as a polymer, is directed at the part or it is submerged. This transforms the structure to martensite, which is much harder than the prior structure.
A popular, modern type of induction hardening equipment is called a scanner. The part is held between centres, rotated, and passed through a progressive coil which provides both heat and quench. The quench is directed below the coil, so any given area of the part is rapidly cooled immediately following heating. Power level, dwell time, scan (feed) rate and other process variables are precisely controlled by a computer.
There is a direct correlation between hardness and wear resistance. The wear resistance of a part increases significantly with induction hardening, assuming the initial state of the material was either annealed, or treated to a softer condition.
The compressive stress (usually considered a positive attribute) is a result of the hardened structure near the surface occupying slightly more volume than the core and prior structure.
As with any process producing a martensitic structure, tempering will lower hardness while decreasing brittleness.
Typical case depth is .030” - .120” which is deeper on average than processes such as carburizing, carbonitriding, and various forms of nitriding performed at sub-critical temperatures. For certain projects such as axels, or parts which are still useful even after much material has worn away, case depth may be up to ½ inch or greater.
Areas with post-welding or post-machining stay soft - very few other heat treat processes are able to achieve this.
Example: a shaft 1” ? x 40” long, which has two evenly spaced journals, each 2” long requiring support of a load and wear resistance. Induction hardening is performed on just these surfaces, a total of 4” length. With a conventional method (or if we induction hardened the entire length for that matter), there would be significantly more warpage.
The most popular steel utilized for parts to be induction hardened is 1045. It is readily machinable, low cost, and due to a carbon content of 0.45% nominal, it may be induction hardened to 58 HRC +. It also has a relatively low risk of cracking during treatment. Other popular materials for this process are 1141/1144, 4140, 4340, ETD150, and various cast irons.